162 USE OF SUNLIGHT FOR PROJECTION lCH. VI 



slides. Care must be taken to have no direct light fall on the screen 

 except that from the lantern, but the room can have sufficient 

 diffused light to take notes comfortably (see also Ch. XII, 

 605-608). 



261. Size of the room and the screen. By using a condenser 

 of proper size and of a focal length adapted to the projection 

 objective, there is no practical limit to the possibilities of projection 

 with sunlight. 



262. Turning on and off the light. For shutting out the sun- 

 light one can use a metal shield between the mirror and the con- 

 denser or one can use the objective shield (fig. 14 and 62). The 

 first method is preferable, for there will be less heating of the 

 apparatus. 



TROUBLES 

 263. The troubles with sunlight are: 



1 . The difficulty of keeping the beam of sunlight in a constant 



direction. With the porte-lumiere one must be con- 

 stantly on the alert to make the slight adjustments of the 

 mirror necessary. 



2. The clock-driven heliostats, if well made and regulated 



accurately, should give no trouble when they are prop- 

 erly setup. 



If a person is fortunate enough to live near an astronomical 

 observatory and can get the help of the astronomer in charge he can 

 learn to overcome difficulties that seem to be insurmountable when 

 working alone. The apparatus of an observatory is also of first 

 rate quality, and it helps any worker to know what good apparatus 

 looks like. 



264. Lack of sunlight. This is the one great trouble. Of 

 course it is not available at night anywhere. And in the most 

 thickly populated regions where projection apparatus is used there 

 is liable to be so much cloudy weather that sunlight is not available 

 even in the daytime during much of the year. Smoke also obscures 

 the sun when clouds are absent. 



